Twenty miles per hour speed zones reduce the danger to pedestrians and cyclists

Dear Editor

I would like to confirm that there are no increased health risks posed by higher petrol consumption or car emissions. Imperial College, London conducted research for the City of London on the effects on emissions of speed limit reduction from 30mph to 20mph. Their conclusion was "It was therefore concluded that air quality is unlikely to be made worse as a result of 20mph speed limits on streets in London." See p66 https://www.cityoflondon.gov.uk/business/environmental-health/environmen...

This was conducted using real-world cycles of acceleration and deceleration within a city environment. In particular the report showed an an 8% reduction in NOx and PM10 emissions for 20mph compared to 30mph in diesel cars.

For a discussion of how emissions and fuel usage are influenced by reduced speed limits see http://www.20splenty.org/do_emission_increase

The most important factor in real world emissions from cycles of 0-20mph and 0-30mph is the energy used to repeatedly accelerate to the terminal speed. As this is proportional to the terminal speed squared then reaching 30mph requires 2.25 times the energy required to reach 20mph.

Of course, there are additional advantages from creating a slower (and less hostile) environment for pedestrians and cyclists in that it encourages previous drivers to switch to walking or cycling thereby eliminating the emissions from their previous driving of a motor vehicle.